Thermoelectric couple and method of making same



G.HUNRATH Jan. 27, 1953 THERMOELECTRIC COUPLE AND METHOD OF MAKING SAMEFiled Aug. 2, 1950 V MM:

Patented Jan. 27, 1953 THERMOELECTRIC COUPLE AND METHOD OF MAKING SAMEGeorge Hunrath, Asbury Park, N. J., assignor to the United States ofAmerica as represented by the Secretary of the Army Application August2, 1950, Serial No. 177,314

(Granted under Title 35, U. S. Code (1952),

sec. 266) 2 Claims.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

This invention relates to thermocouple generators and more particularlyto an improved method of making them.

As is well known, thermocouples are formed from positive and negativeelements connected at one end to form a hot junction, the opposite freeends of the elements forming the so-called cold junction.

Among the difi'erent elements for thermocouples that have been producedare alloys formed from zinc and antimony in predetermined proportionswith small percentages of additional agents. These alloys have beenfound to be highly eflicient when used as the positive element of thethermocouple in the conversion of thermal energy to electrical energy.The composition of such an alloy is described in U. S. patent to Telkes,No. 2,229,482 issued January 21, 1941.

In employing this particular alloy as the positive element certaindifiiculties have been encountered in its fabrication and assembly. Theusual procedure which is generally followed is to prepare this alloy ina crucible or container under a protective atmosphere such as nitrogenor helium and to cast the alloy into a suitable mold. In this method ithas been found extremely diflicult to prevent the rapid oxidation of thealloy even under this protective atmosphere. Oxidation of the metalsduring casting usually results in a higher electrical resistance of thealloy and in high resistance contacts to external connections, thuslowering its thermoelectric efliciency.

Furthermore, the Zn-Sb alloy is mechanically weak and brittle andtherefore the fabrication and construction of the thermocouple shouldavoid any mechanical strain, especially when the alloy is in anoperative position as one element of the thermocouple. It has also beenfound that the Zn-Sb element described is somewhat unstable to theextent that after it has been in use for a period of time as thepositive element of the thermocouple an extrusion of zinc and oxidationof the elements take place at the cold junction, due to thermolyticaction and exposure of the elements to the atmosphere, with anaccompanying decrease in efiiciency of the thermocouple.

It is therefore an object of this invention to provide a hermeticallytight thermocouple as- 2 sembly resulting in a thermocouple having highefliciency and long life.

It is a further object of this invention to provide a method ofassembling the elements of a thermocouple with a minimum of oxidation ofone of these elements.

It is another object of this invention to provide an adequate housingfor one of the elements of this thermocouple.

Other objects of this invention will become apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich:

Fig. 1 is a view in section of one method of assembling thethermocouple;

Fig. 2 is a view in section of a thermocouple generator utilizing theassembly as shown in Fig. 1.

Fig. 3 is a perspective view of the thermocouple of Fig. l assembled asa thermo-electric generator.

Referring to Fig. 2 there is shown a thermocouple assembly Hl consistingof a glass cylinder l2 having metal ends I! and IS in the form of caps.The metal ends l4 and I6 are fitted into recesses It at both ends ofglass cylinder 12 which are of suflicient depth to insure a satisfactoryhermetic and airtight seal between the metal caps and the glass cylinderl2. The metal caps [4, I6 are preferably made of a metal compositionknown as Kovar and the glass cylinder I2 is made of a glass known asCorning No. 705-2. It is to be understood that for the purposes of thisinvention the composition of the glass cylinder i2 and the metal capsl4, is need be only such that the metal caps M, It and the glasscylinder I2 have substantially the same coefficient of expansion andthat the metal caps l4, is be of electrically conductive material whilethe glass cylinder l2 be of suitable insulating and heat resistantmaterial.

At the hot junction 20 a strip of Kovar, Copel or Constantan or asimilar metal 24 having a negative thermal E. M. F. to the alloy ormetal to be placed in the assembly Ill is welded to the metal cap l6 andsimilarly at the cold junction 22 another strip of conductive metal 26preferably copper is welded to metal cap l4. Within the assembly I ll,itself, is housed an alloy or metal 23 having a positive thermal E. M.F. to metal 24.

Thus, in the particular embodiment of the invention as shown in Fig. 2,the thermocouple assembly Ill consists of an hermetically tightcontainer formed by side walls l2 and end caps I4, IS, a positivethermo-electric element 28 completely filling the container, a. negativethermoelectric element II, It disposed at the ends of the container andin good electrical contact with positive element 28; the disposition ofelement l4, l6 being in the form of metal caps, as shown, so as to makean airtight hermetic seal at the open ended insulated side walls 12, andat the same time forming the hot junction 20 and the cold junction 22 ofthe thermocouple.

Referring to Fig. 1 this invention is illustrated by reference to aparticular method for producing the desired thermocouple. The glasscylinder l2 and small beakers or other suitable vessel not shown areplaced in a vacuum furnace, not shown. One beaker contains zinc and theother beaker contains antimony in the predetermined portions mentionedhereinbefore. The metals are then melted in the vacuum furnace, thuseliminating any oxidation that would occur by the use of ordinarymethods of casting. The beakers are then tilted to pour the moltenmetals into cylinder I2 through entrance tube 28. After cooling, theentrance tube 28 is ground off flush with the cylinder [2 and thensealed by the metal caps l4, l6. Conductor 26 is then welded to metalcap M and the negatively thermal metal 24 is welded to metal cap IS. Theentrance tube 28 need not necessarily be in the position as shown butmay be placed at any convenient point on the container It so as toaiford a means for pouring the molten metal into the container.

Instead oi casting the Zn-Sb alloy as described above the Zn-Sb alloycan be produced by placing well mixed granules of zinc and antimony inthe predetermined proportions into assembly III which already has metalcap I6 sealed into position, then sealing the remaining open end ofcylinder l2 with metal cap l4, and then subjecting the mixed granules tosuflicient heat in a vacuum furnace or any other suitable oxide-freeheating means to effect a fusion of the granules without oxidation.Furthermore, it has been found that the fusion of the mixed granules ina vacuum efiectuates a physically and electrically tight bond betweenthe positive thermo-electric element 28 within the assembly and thenegative thermoelectric element Hi, It forming the ends of the assembly.Pursuing either method the assembly I is utilized both as the mold forcasting the Zn-Sb alloy and as an hermetically tight housing for thealloy during the functioning of the thermocouple as an operative device.

The metal strips 24, 26 are in the shape as shown in order to facilitatethe series electrical connections of each thermocouple when used as athermo-electric generator, as shown in Fig. 3.

This method of assembling a thermocouple may be used in makingthermocouples with other ele- 5 ments than those mentioned. For example,an alloy of tellurium may be substituted for the Zn-Sb alloy.

By means of the method described herelnbefore the oxidation of the Zn-Sballoy during casting is eliminated resulting in a lower electricalresistance and a higher electro-motive force. An adequate and ruggedhousing is provided for the brittle alloy. The hermetically tight sealat the junction points of the assembly prevents the extrusion of zincdue to thermolytic action and also prevents oxidation of the elements attheir junction during use, all of which results in a thermocouple havinga relatively high efiiciency and long life.

While the invention has been described with reference to severalparticular embodiments, it will be understood that various modificationsmay be made therein within the scope of the appended claims withoutdeparting from the field of invention.

I claim:

1. In a thermocouple having first and second electrically dissimilarelements, a housing having side walls of an insulating material and endwalls of an electrically conductive material having substantially thesame coefficient of expansion as said side walls, said first elementsubstantially filling said housing and electrically contacting said endwalls, said end walls forming said second element, the edges of said endwalls protruding into and sealed within said side walls hermeticallysealing said first element within said housing, and a strip of saidsecond element welded to one of said end walls forming the hot junctionof said thermocouple.

2. In the thermocouple defined by claim 1, said side walls beingcylindrical in shape and said end walls being cup shaped.

GEORGE HUNRATH.

REFERENCES CITED Ihe following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 434,640 Mestern Aug. 19, 1890773,838 Wightman Nov. 1, 1904 2,144,558 Bahls Jan. 17, 1939 2,280,137Wiegand Apr. 21, 1942 2,289,152 Telkes July '1, 1942 2,290,902 WiegandJuly 28, 1942 2,480,404 Findley et al Aug. 30, 1949 2,490,196 Beach Dec.6, 1949 FOREIGN PATENTS Number Country Date 168,977 Great Britain Sept.9, 1921

